Generally, in the field of the creation of two-dimensional cell animations, the creation of the motion of a character is divided into “original images” and “moving images”. An “original image” is a picture in which a pose that determines the motion as a whole is drawn, and a “moving image” is a picture produced by interpolation between one “original image” and another.
Meanwhile, three-dimensional computer graphics are now widely used as another method for modeling. In three-dimensional computer graphics images, after the creation of data known as “key frames”, data between key frames is created automatically by means of spline interpolation and the like. When a “key frame” is regarded as an “original image” in the cell animation, it is said that working methods which are adopted in both cases are highly analogous.
However, as matters now stand, three-dimensional computer graphics modeling has not yet reached the expressive capacity of two-dimensional cell animation. The primary reason is said to be found in the work operation which is presented by way of example hereinbelow.
For example, in the creation of the motion of a human-type model, in three-dimensional computer graphics, progress is being made in the shape creation of human-type models in three dimensions, but as to the method for bringing movement to each of the parts of the human-type model, such as the hands and feet and head thereof, as matters now stand, in a broad classification, only motion capture and methods for the computational processing of 3-D rotational data are employed. Motion capture only provides the motion of a real person, and therefore sometimes limits the free expression of the animator. Further, the human-type model is a structure comprising a multiplicity of links and therefore possesses complexity and a large degree of freedom. The links are structural units having a joint section constituting a center of rotation, and a long section that extends from the joint section. The links can also be constituted by the joint section alone.
FIG. 19 shows an example of a simple human-type model that is constituted by a plurality of links. In FIG. 19, the joint section 11 of each link 10 is shown as a circular section, and the long section 12 extending from the joint section 11 is shown as a linear section. There have been problems in controlling the motion of such a human-type model by means of conventional computational processing methods (the simple inverse kinematics method, for example). For example, with an animation creation program that uses the conventional inverse kinematics method, it is not possible, when one link is moved, to automatically determine the positions of the other links so that the human-type model retains a posture that is as natural as possible. More precisely, a conventional animation creation program presets the joint sections for all the links to be either movable or fixed. Then, there has been the inconvenience that, when any one link is selected and the position of this link is moved (or rotated), the positions of the other movable links move upon exceeding the rotational range of the joint sections thereof, whereas the positions of the joint sections which are fixed do not move, which makes the posture of the human-type model unnatural, and that a selected link cannot move to the desired position because the other links are fixed. In this case, the animator was required to perform an operation to cancel the fixation of every one of the joint sections, and, in order to create a posture for the human-type model, to create the desired posture by moving each of the links. Consequently, the operation of a conventional animation creation program was complicated and inefficient, and the program was difficult for the animator to use.